But they do not seem to have been able to spread the doubt much beyond their own hardcore constituency.

Among Tea Party supporters, Republican and Independent, only 30% see evidence of global warming and 11% accept it is manmade, according to the poll.

Among those opposed to the Tea Party, Republican and independent, a majority, 56%, see concrete proof of global warming and 28% say it is caused mainly by humans.

Among Americans overall the number of people who believe there is concrete proof of global warming rose to 63%, from 59% last year.

More than three-quarters of Democrats see evidence of climate change.

Among independents 63% believe in climate change, up 10 points from two years ago.

Some 38% of Americans believed the climate was changing because of human activity, a rise again over last year when 32% believed in manmade global warming.

The numbers of Americans are still not as high they were in 2006, the year Al Gore released his award-winning Inconvenient Truth documentary. At that time more than three-quarters, 77%, of Americans said they believed in climate change, and about half believed it was man-made. More than 40% viewed climate change as a very serious problem.

But the numbers are beginning to head back up from that great slough of denial of the last two years, when the Kochs began channelling more money to climate doubting thinktanks and astro-turf groups.

Except, that is, among conservative Republicans – the target audience of the Tea Party campaigns. Just 31% see clear evidence of climate change.

The poll, released on Wednesday, was conducted on 9-14 November among 2,001 adults.